Thermal switch assembly having juxtaposed auxiliary heater



Feb. 4, 1969 I .T. G. SCHEID m, ETAL ,4 3

THERMAL SWITCH ASSEMBLY HAVING JUXTAPOSED AUXILIARY HEATER Filed Aug. 16,1965

Sheet bf 2 IN VENTOR 777 eoaore 6'. Gabe/UH 5's derv'cfi 6'. LSc'AuaneAs By% ATTORNEYS Feb- 1969 T. G. SCHEID lll, ETAL 3,426,306

THERMAL SWITCH ASSEMBLY HAVING JUXTAPOSED AUXILIARY HEATER Filed Aug; 16, 1965 Sheet 2 of 2 INVENTOR m" A-WKW A T RNEYS United States Patent 6 Claims ABSTRACT OF THE DISCLOSURE An auxiliary heater externally engages a thermostat casing, the auxiliary heater being jacketed with foil and held in such engagement by shrunken irradiated plastic which plastic also provides strain relief for all wiring by having a heat-seal with its insulation, and which plastic holds internal wiring in non-crossing relation.

This invention relates generally to an electrical switch assembly, and more specifically to a thermal switch assembly which is especially suited for use in an electrical heating circuit such as is employed in an electrical heating pad.

Although the principles of the present invention may be included in various devices, a particularly useful application is made in electrical heating pads.

When electrical heating pads have been provided with thermal switches of prior types, various diificulties have arisen. Where the thermal switch assembly includes an auxiliary heater, the operation of the auxiliary heater has not been efficient. Moreover, such auxiliary heating element has crossed over a thermostat leadwire, and when such a construction is employed in an electrical heating pad, and a force is applied thereto (such as the weight of a person sitting on such pad) there has been a tendency for failure to occur at the point where the auxiliary heater crosses over the thermostats leadwire. Also, the mode of enclosing or protecting a thermostat within a heating pad has previously been bulky. The combination of circumstances thus referred to has caused a relatively large initial overshoot, coupled with a relatively wide differential of cycling. By way of example, when first energized, such prior-art construction would undergo a temperature rise so that the auxiliary heater would reach a temperature of 330 -F. before the switch would first open, and thereafter cycle over a 30-degree range, such as from 180 F. to 230 F. Such construction had the further effect of creating a relatively large diameter hotspot where the thermostat having the heater was located, and would also be the cause of burning if the relatively high overshoot of temperature took place while the pad was in a wadded configuration.

In accordance with our invention, we have provided a construction which enables a great reduction in wattage employed in the auxiliary heater, which thereby eliminates or minimizes the previous large diameter hotspot. This construction provides less overshoot in temperature for the initial cycle, and the device cycles over a narrower temperature differential. Moreover, somewhat longer life is obtained. Still further, n0 temperature is reached which could produce scorching, and therefore even if the pad be wadded, no harmful effect will take place.

3,426,306 Patented Feb. 4, 1969 Accordingly, it is an obejct of the present invention to provide an improved thermal switch assembly.

A further object of this invention is to provide a thermal switch assembly employing an auxiliary heater which will maintain a thermal bias with greater efficiency than that which has been previously done.

A still further object of the present invention is to provide a thermal switch for a heating pad of such construction as to avoid creation of a large diameter hotspot.

Another object of the present invention is to provide a thermally biased switch of such consructtion as to be unable to attain a temperature which would scorch textile materials.

Yet another object of the present invention is to provide a thermal switch which will operate with a relatively low differential between cut-in and cut-out.

A still further object of the present invention is to provide structure by which strain relief is aiforded t0 wiring leading to an electrical switch.

Many other advantages, features and additional objects of the present invention will become manifest to those versed in the art upon making reference to the detailed description and the accompanying sheets of drawings in which a preferred structural embodiment incorporating the principles of the present invention is shown by way of illustrative example.

ON THE DRAWINGS FIGS. 1-8 and 11 illustrate successive steps in constructing a thermal switch assembly, provided in accordance with the principles of the present invention;

FIG. 9 is an enlarged cross-sectional view taken along line IXIX on FIG. 8;

FIG. 10 is a diagrammatic view of means for producing a heat seal on the switch assembly, such heat seal being shown in enlarged cross section and taken along line XX of FIG. 8; and

FIG. 12 is a circuit diagram of an electric heating pad having a heater circuit employing the thermal switch assembly of FIG. 11.

AS SHOWN ON THE DRAWINGS The principles of the present invention are particularly useful when embodied in a preferred form of encapsulated thermal switch assembly such as is illustrated in FIG. 11, generally indicated by the numeral 15. The components which form a part of the switch assembly 15, and their manner of assembly can be readily ascertained by reference to FIGS. 1-8.

A resistance wire 16 is wound in spiral fashion onto a core 17 which preferably is flexible to provide an auxiliary heater generally indicated at 18. The auxiliary heater 18 is connected to the conductor of an insulated wire 19 by a suitable clip 20 known in the trade as an autoband. Thereafter, a piece of adhesive tape 21 which comprises electrical insulation is wound about a major portion of the auxiliary heater 18, about the Connecting clip 20, and the end of the insulation of the wire 19. The beginning of such winding or wrapping is illustrated in FIG. 1, and when completed, it has the appearance shown in FIG. 2. The insulation on the wire 19 comprises plastic, such as vinyl plastic, and may typically have a temperature rating of C.

Thereafter, as shown in FIG. 3, a piece of metal foil 22 is wound about the auxiliary heater 18, and is spaced and insulated electrically thereform by the electrical insulative tape 21. The foil 22 extends from the connecting clip to a point which is substantially flush with the insulative tape 21. In the ultimate assembly, the heater wire 16 immediately adjacent to the edge 23 of the foil 22 will have the same electrical potential, and therefore, it is not necessary that the edge 23 of the foil 22 be insulated from the resistance wire 16. When the foil 22 has been wound onto the insulative tape 21, the appearance obtained is that shown in FIG. 4.

Thereafter, as shown in FIG. 5, a further insulated wire 24 is connected to the resistance wire 16. To this end, the insulation is removed to provide an uninsulated portion 25 which is disposed against the resistance wire 16, and which is held in such place by a further metallic connector clip 26 which is identical to the clip 20. The wire 24 is insulated with plastic having the properties of the insulation of the wire 19.

As shown in FIG. 6, a switch or elongated thermostat 27 is provided having a terminal 28 which forms a part of its case, and a terminal 29. An example of such a thermostat is shown in the US. patent issued to Porter, No. 2,627,003. The subassembly of FIG. 5 is connected to the terminal 28 by soldering the end portion 25 of the wire 24 thereto. Similarly, a further wire having plastic insulation of the properties described for the wire 19 is connected in any convenient manner to the terminal 29. A plastic sleeve 31 is slipped over the connection between the terminal 29 and the wire 30 and is of a length to embrace a portion of the terminal 29 and a portion of the insulation of the wire 30. The plastic sleeve 31 preferably comprises irradiated polyvinylchloride which is then shrunk so as to form a tight fit with the terminal 29 and the insulation of the wire 30 as shown in FIG. 7.

Irradiated polyvinylchloride is commercially available. It is understood that polyvinylchloride tubing is expanded and while expanded, it is irradiated by a radioactive source. Such treatment serves to hold-the molecules of the plastic in the expanded position. However,

" they can be released and permitted to return to their original position by raising the temperature of the tubing, by means of an oven or an infrared heater, to 250350 F. Irradiated Mylar and irradiated polypropylene have similar physical properties.

After the plastic sleeve 31 has been shrunk by application of heat thereto, the auxiliary heater is positioned along one of the lengthwise-extending surfaces of the thermostat 27 as shown in FIG. 7. When so arranged, the foil 22 lies in intimate contact with the metallic case of the thermostat 27, and extends only along one side thereof. Preferably, this relationship should be maintained, and the auxiliary heater 18 should not be so long as to be able to engage the wire 30. The assembly thus far described is now ready for encapsulation. If there is to be intervening handling, it may be convenient to provide a small piece of adhesive tape 32 having properties like that of the tape 21 to hold the auxiliary heater 16 tem porarily in place with the foil 22 disposed against the case of the thermostat 27. If this be done, the tape 32 is wound about the thermostat 27 and the foil 22 as shown in broken lines in FIG. 8.

As shown in FIG. 9, thereafter a plastic tube 33, preferably also comprising irradiated polyvinylchloride is slipped over the subassembly of FIG. 7 with the wires 24, 19, and 30 projecting from the ends thereof. Both of the ends of the plastic tube 33 are flattened and heat-sealed together and sealed to the insulation of the wires 19, 24, and 30. The plastic sleeve 31 also has part of the portion which is shrunken onto the insulation of the wire 30 heat-sealed into the seal which closes the adjacent end of the tube 33.

To accomplish such a seal, there is provided a pair of retractible dies 34, 35 which have a thickness corresponding to the width of the seal to be made. The confronting edges of the dies 34, 35 have indentations or dimples 36-39, and the wire 19 is moved so as to register with the dimples 36, 38, while the wire 30 is similarly moved so that the plastic sleeve 31 registers with the dimples 37, 39. The dies are then brought together, and the plastic material therebetween is heated by any suitable means, such as a radio frequency source 40. FIG. 10 illustrates the cross sectional configuration of the finished heat seal or bond which is thus produced along the line X-X of FIG. 8. By means of similar tooling, the heat seal shown in enlarged cross section in FIG. 9 is produced at the opposite end of the tube 33.

Thereafter, the entire assembly is subjected to heat, such as by means of an oven or an infrared lamp, to raise the temperature of the plastic tube 33 to one Within the range of 250350 F. so as to enable the irradiated plastic tubing to shrink and to form a tight fit with the components against which it shrinks as shown in FIG. 11.

Such shrink fit brings the auxiliary heater 18 into close proximity with the adjacent longitudinal side of the thermostat 27, and more specifically forces the foil 22 directly against the case of such thermostat 27.

In the course of such shrinking, it becomes desirable to enable air to escape. To this end, in the formation of the heat seal shown in FIG. 9, tooling is employed corresponding generally to that of FIG. 10 wherein each of the dies has two dimples. However, only the wire 24 is dressed or positioned to fit into one of the coacting sets of dimples, leaving the other set of dimples with no wire therein. Such sealing has the effect of producing a very small air vent 41, shown in enlarged form in FIG. 9.

Thus a bond is formed between the plastic insulation of the wire 24 and the tube 33 at a point indicated by the numeral 42. A similar bond is formed at 43 between the insulation of the wire 19 and the tube 33. A threeway bond is thus formed at the point 44 between the insulation of the wire 30, the plastic sleeve 31, and the plastic tube 33. The bonds 42-44 thus form a part of heat-sealed end closures of the tube 33. These bonds enable the tube 33 to hold the various wires or wire means in non-crossing relation to each other. Thus if someone should sit on the structure as shown in FIG. 11 when the same is in a heating pad, there would be no tendency for short circuits to form.

Irradiated polyvinylchloride is capable of undergoing a relatively large shrinkage in a direction which corresponds to the diameter or radius of the tube 33. It also has about a ten percent shrinkage in its axial direction. Thus the shrinkage of the tube 33 draws the end closures thereof together, and therefore tends to compress the wiring slightly, which will deflect to the extent necessary, such as at 45. However, if a tension be thereafter applied between the Wires 24 and either 19 or 30, the bonds at 42-44 will transmit such tension forces through the sleeve 33, and thus apply substantially no tension to any of the internal wiring connections. Moreover, the shrunken sleeve 31 provides a strain relief which protects the connection between the wire 30 and the terminal 29.

The thermal switch assembly of FIG. 11 may be connected into a heater circuit such as shown in FIG. 12, such circuit forming no part of the present invention. A manual selector 46 is adjusted to alter the proportion of current which passes through the auxiliary heater 18. Such current is converted to heat in the spirally wound heater wire 16 which escapes radially through the thin electrical insulation 21 and is collected by the foil 22 around the entire periphery, such heat being uniformly distributed by the foil and being rapidly transferred by conduction from the foil to the metal case of the thermostat 27. In the low position, a maximum amount of current flows through the auxiliary heater wire 16, thus causing the thermostat 27 to open in response to a minimum amount of additional heat from the heater 47 in the heater circuit. With the manual control 46 in the high position, the auxiliary heater wire 16 of the auxiliary heater 18 is out of the circuit, and the thermostat 27 functions solely in response to heat from the heater 47, as does a further safety thermostat 48. The thermostat 48 may be encapsulated in accordance with the principles taught herein. As the possible axial shrinkage of the tube 33 is about ten percent of the possible diametral shrinkage per unit length, the same tooling and tube 33 may be employed for this purpose.

The construction in accordance with this invention enables approximately a fifty percent reduction in wattage utilized by the auxiliary heater 18. Such reduction in wattage avoids the creation of an objectionable hot spot in the heating pad 49 directly adjacent to the thermal switch assembly 15. With this construction, the thermostat 27, calibrated to close at a temperature of 180 F., will reopen at a temperature of 190 F. during normal cycling, while prior constructions, employing the same thermostat 27, would not shut off until a temperature of 230 F. was reached. Similarly, the initial temperature overshoot has been reduced from 330 F. to 240 F. The elimination of the high temperature overshoot has greatly extended the life of heating pads employing this assemply. A ISOO-hour heating pad life was previously not uncommon, while after such duration of usage with the present construction, any tendency for deterioration to set in has been negligible. The present construction is inexpensive, is small, provides adequate electrical insulation for the auxiliary heater, and minimizes thermal insulation between the auxiliary heater 18 and the thermostat 27. Materials of low specific heat have been employed, which further minimize the operating temperature differential. While all the foregoing advantages have been obtained, the size of the assembly has been reduced from that of prior constructions, and the amount of wire utilized, both as an auxiliary heater 18 and for conducting purposes, has been reduced. Moreover, even though there has been the saving of wire, a structure is provided in which there is no appreciable fiexure at the solder joints provided between the wire 25 and the terminal 28 and provided between the wire 30 and the terminal 29.

Although various minor modifications might be suggested by those versed in the art, it should be understood that we wish to embody Within the scope of the patent warranted hereon all such embodiments as reasonably and properly come within the scope of our contribution to the art.

We claim as our invention:

1. A thermal switch assembly for a heater circuit comprising:

(a) an elongated thermostat including at least one terminal;

(b) insulated wire means electrically connected to said thermostat for connection in the heater circuit and including a plastic-insulated wire connected to said terminal;

(c) an auxiliary heater circuit including an auxiliary heater disposed against said thermostat along the length of said thermostat and electrically insulated therefrom,

(d) a plastic sleeve enclosing the connection between said insulated wire and said terminal;

(e) a plastic tube enclosing said thermostat, said auxiliary heater, and said sleeve; said plastic tube, said plastic sleeve, and the plastic insulation of said wire having a bond uniting them together; and

(f) the plastic of said sleeve and the plastic of said tube being a material having the physical properties of irradiated polyvinylchloride, said sleeve having a shrink fit against the connection which it encloses by which said thermostat is mechanically connected to said bond, and said tube having a shrink fit against said thermostat and said auxiliary heater by which the engagement between said thermostat and said heater is maintained.

2. A switch assembly comprising:

(a) a switch including at least one terminal;

(b) plastic-insulated wire means electrically connected to said switch for connecting it in a circuit, and including a wire having a connection with said terminal;

(c) a plastic sleeve enclosing the connection between said insulated wire and said terminal;

(d) a plastic tube enclosing said switch and said sleeve;

(e) a bond uniting said plastic tube, said plastic sleeve and the plastic insulation of said wire means, said wire means projecting from opposite ends of said tube; and

(f) the plastic of said sleeve and the plastic of said tube being a material having the physical properties of irradiated polyvinylchloride, said sleeve having a shrink fit against the connection which it encloses by which said switch is mechanically connected to said bond, and said tube having a longitudinal shrink between said bond and said switch by which applica tion of a tension on said connection is precluded.

3. A thermal switch assembly for a heater circuit comprising:

(a) an elongated thermostat adapted to be connected in electrical series with the heater circuit and having a case;

(b) an auxiliary heater; and

(c) flexible metal foil electrically insulated from and jacketing said auxiliary heater, said foil being disposed along and as a separate element against only one lengthwise-extending surface of the case of said thermostat.

4. A thermal switch assembly for a heater circuit comprising:

(a) an elongated thermostat adapted to be connected in electrical series with the heater circuit and having a case;

(b) an auxiliary heater;

(0) flexible metal foil electrically insulated from and jacketing said auxiliary heater, and disposed along and as a separate element against a surface of the case of said thermostat; and

(d) a plastic heat-shrunk tube surrounding said thermostat and said foil and urging said foil snugly against said thermostat case.

5. A thermal switch assembly for a heater circuit comprising:

(a) an elongated thermostat having insulated wire means for connecting said thermostat in the heater circuit;

(b) an auxiliary heater disposed against said thermostat along the length of said thermostat, and having insulated wire means for applying power thereto; and

(c) a plastic tube of a material having the physical properties of irradiated polyvinylchloride enclosing said thermostat and said auxiliary heater and from at least one end of which said wire means project,

(1) said tube having heat seals closing its ends through at least one of which seals said wire means pass in sealed relation and by which said wire means are held;

(2) said tube having a shrink fit with said thermostat and said auxiliary heater by which the engagement between said thermostat and said heater is maintained; and

(3) vent means extending into said tube for insuring a maximum possible shrinkage of said tube onto said auxiliary heater and said thermostat.

6. A switch assembly comprising:

(a) a switch having insulated wire means by which it may be connected in a circuit;

(b) a plastic tube enclosing said switch and from the ends of which tube said insulated wire means extend, said tube being of a material having the physical properties of irradiated polyvinylchloride;

(c) said tube having heat seals closing its ends through which seals said wire means pass, the insulation of said wire means forming a part of said seals whereby said wire means are both held by and sealed to said tube;

((1) said tube having a longitudinal shrink between its ends which applies a compressive force to the wire means and said switch between said seals, whereby application of a tension through said switch is precludedj and (e) vent means extending into said tube for insuring a maximum possible axial shrinkage of said tube to draw said heat seals toward said switch.

References Cited UNITED STATES PATENTS 3,280,307 10/1966 Ramey et a1. 219540 3,119,926 1/1964 Mills et a1. 219212 2,675,445 4/ 1954 Youhouse. 1,127,373, 2/1915 Read 219540 2,971,073 2/1961 Eisler 338212 X Eisler 219549 X Kirkpatrick et a1. Kirkpatrick et al. Wells 200-138.6 X

Moyer. Dykes 219-212 X FOREIGN PATENTS France.

OTHER REFERENCES Alphlex: Shrinkable Tubing, received in the Patent Otfice June 21, 1961, pp. 1-4.

US. Cl. X.R. 

